Dead time is necessary for the coupled power switches to prevent shoot-through, especially in the modular multilevel converters (MMCs) with a large number of power switches. This paper proposes a dead-time effect suppression strategy for MMCs with nearest level modulation. The operational principles of MMCs are first analyzed. According to the operational features of MMCs, the method that removes a switching signal from the coupled switches and the reduced switching frequency voltage balancing algorithms (RSFVBAs) are mixed in the proposed method. In the intervals that are furthest away from the zero-crossing points (ZCP) of arm currents, the single switching signal method can completely eliminate the dead-time effect (DTE). Alternatively, the DTE is suppressed by the RSFVBA in intervals that are close to the ZCP. By the combination of the two methods, the dependence of the DTE suppression method on currents is reduced and the influences of ZCP are also released without degrading the normal operation performance of MMCs. Moreover, the output performance of MMCs is improved and the voltage stress on the arm inductor dramatically decreases. Finally, the validation of the method is verified by the simulation results with the professional tool Matlab/Simulink.

Due to the large number of submodules (SMs), and modular multilevel converters (MMCs) in high-voltage applications, they are usually regulated by the nearest level modulation (NLM). Moreover, the large number of SMs causes a challenge for the fault diagnosis strategy (FDS). This paper proposes a currentless FDS for MMC with NLM. In FDS, the voltage sensor is relocated to measure the output voltage of the SM. To acquire the capacitor voltage and avoid increasing extra sensors, a capacitor voltage calculation method is proposed. Based on the measurement of output voltages, the faults can be detected and the number of different-type switch open-circuit faults can be confirmed from the numerous SMs in an arm, which narrows the scope of fault localization. Then, the faulty SMs and faulty switches in these SMs are further located without arm current according to the sorting of capacitor voltages in the voltage balancing algorithm. The FDS is independent of the arm current, which can reduce the communication cost in the hierarchical control system of MMC. Furthermore, the proposed FDS not only simplifies the identification of switch open-circuit faults by confirming the scope of faults, but also detects and locates multiple different-type faults in an arm. The effectiveness of the proposed strategy is verified by the simulation results.